Chemical vapor deposition (CVD) method has been used in the deposition of silicon dioxide films onto semi-conducting substrates. In the method, a gas containing the structural elements of the film material to be formed is fed into a chamber, followed by heating the gas to induce a chemical reaction to deposit the desired film on the semi-conducting substrate. In a conventional CVD method, a silicon dioxide film is frequently deposited by a reaction between silane and oxygen. The method enables the formation of a silicon dioxide film at a relatively low temperature.
In the fabrication of an advanced semiconductor device that requires a steep step coverage by a silicon dioxide film, the conventional CVD method of deposition by silane and other reactant gases was found to be inadequate, i.e., presenting a step coverage problem. In order to solve the step coverage problem, other methods have been developed. One of such methods is the utilization of a reactant of tetra-ethoxy-ortho-silicate (TEOS) which has better coverage characteristics than silane, although TEOS must be processed at a high reaction temperature due to its low reactivity.
In the high temperature processing of TEOS, the pyrolysis of TEOS for obtaining an oxide film is similar to the decomposition process of silane in forming polysilicon. A carrier gas, typically an inert gas such as nitrogen, can be used to bubble through liquid TEOS to provide a gas mixture of controlled TEOS partial pressure in the reaction chamber. Since TEOS consists of a silicon atom that is symmetrically bonded to four OC.sub.2 H.sub.5 groups, the decomposition of TEOS at a high temperature, i.e., between 650.degree. C. and 750.degree. C. results in silicon dioxide and by-products of organic and organosilicon compounds. For instance, the pyrolysis reaction of TEOS can be represented by: EQU Si(C.sub.2 H.sub.5 O).sub.4.fwdarw.SiO.sub.2 +by-products
The deposition rate of SiO.sub.2 depends on the TEOS partial pressure presented in the reaction chamber and the reaction temperature. Oxide films may also be produced by reacting TEOS with oxygen at low pressure and high temperature, i.e., between 650.about.750.degree. C. The reaction produces SiO.sub.2, CO.sub.2 and H.sub.2 O. The TEOS oxide films produced by the LPCVD method exhibit excellent uniformity and step coverage. However, the high decomposition temperature of TEOS precludes its usage for depositing oxide layers over preformed structures that are high temperature sensitive, such as structures formed of aluminum or metal silicides.
In a low pressure TEOS oxide deposition process, the deposition chamber or the chemical vapor deposition chamber must first be evacuated to a very low pressure. For instance, the low pressure between about 1 Pa and about 120 Pa. The evacuation of the chemical vapor deposition chamber is carried out by an electrical dry pump. The conduit that connects between the process chamber and the dry pump and provides fluid communication therein between is controlled by a gate valve for opening or closing the conduit. The gate valve is positioned at a location juxtaposed or neighboring to the process chamber. In such a conventional set up, the conduit, which is frequently a 4-inch diameter stainless steel pipe, is frequently coated with a layer of reaction by-product on its inner wall. The reaction by-product is formed of hydrocarbon chemistry and exists in a white powdery form. Its exact composition varies and is difficult to determine by routine chemical analysis.
The dry pump used to evacuate the deposition chamber is frequently mounted at a floor level substantially below that of the process chamber and operates at the atmospheric pressure. The length of the conduit that connects the process chamber and the dry pump may be as long as 10.about.15 meters which presents ample opportunity for the reaction by-product to deposit on the inner wall when evacuated by the pump. The by-product also cumulates in the dry pump chamber and may even cause the pump to stop functioning due to jamming of the rotors. When jamming occurs, the dry pump stops completely. When the pump stops, the powdery deposit on the interior wall of the conduit and in the dry pump may be siphoned back into the process chamber due to a large pressure differential existed between the dry pump and the process chamber, i.e., at 10.sup.5 Pa vs. at 10.sup.2 Pa. Even though a gate valve is provided in the conduit and is suppose to stop the siphoning or the back steam (as commonly known in a fabrication plant), the gate valve itself may be contaminated by the by-product and thus fails to function properly to stop the siphoning of powder into the chamber. Since a large number of wafers are processed in a modern furnace, i.e., as many as 144 wafers can be processed in a vertical furnace, particle contamination in the furnace can be detrimental to the yield of the fabrication process. The contamination to a process chamber by such by-product must therefore be prevented.
It is therefore an object of the present invention to provide an apparatus for controlling a vacuum system in a process chamber that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for evacuating a process chamber that utilizes a gate valve and a T-shaped cold trap in a conduit connecting the process chamber and the dry pump.
It is a further object of the present invention to provide an apparatus for evacuating a process chamber wherein a gate valve is installed in a conduit providing fluid communication between the process chamber and the dry pump at a location juxtaposed to the process chamber.
It is another further object of the present invention to provide an apparatus for evacuating a process chamber wherein a gate valve and a T-shaped cold trap are installed in a conduit connecting between a process chamber and a dry pump such that the cold trap collects large contaminating particles.
It is still another object of the present invention to provide an apparatus for providing fluid communication between a process chamber and a dry pump with a first gate valve, a T-shaped cold trap and a second gate valve sequentially connected between the process chamber and the dry pump.
It is yet another object of the present invention to provide an apparatus for providing fluid communication between a process chamber and a dry pump wherein a first gate valve is installed in a conduit in close proximity to the process chamber, a second gate valve is installed in the conduit in close proximity to the dry pump and a pressure relief valve is installed bridging across the second gate valve to relieve any pressure build up in the conduit upstream of the second gate valve thus eliminating any high pressure safety problem.
It is still another further object of the present invention to provide a method for preventing chamber contamination by particles from a conduit or a pump by connecting a first gate valve in the conduit immediately adjacent to the process chamber, a second gate valve in the conduit immediately adjacent to the dry pump and a T-shaped cold trap between the first and the second gate valves.
It is yet another further object of the present invention to provide a method for preventing high pressure safety problems in a vacuum conduit system for a process chamber by connecting a first gate valve juxtaposed to the process chamber, a second gate valve juxtaposed to a dry pump and a pressure relief valve connected bridging the second gate valve such that a pressure built up before the second gate valve can be avoided.